I have a problem with my payment gateway on my Opencart (2.3.0.2) installation. These are the symptoms:

1. Both payment gateways (PayPal Express & Sagepay) failed on Friday. When a customer tries to Checkout, they are forced into a loop of where they are returned to the beginning of the checkout process repeatedly.
2. The logs are giving these errors:
2018-05-12 16:39:39 - PHP Notice: Undefined index: L_LONGMESSAGE0 in /var/www/vhosts/example.com/httpdocs/catalog/controller/extension/payment/pp_express.php on line 1345
2018-05-12 16:39:39 - Unable to create Paypal session{"":""}